1. Field of the Invention
This invention relates to high frequency power density measurements; and more particularly relates to the measurement of high frequency, or microwave, power density with portable instruments.
The increasing use of microwave energy for such purposes as consumer and industrial heating ovens, has placed possibly dangerous microwaves energy sources in close proximity to large numbers of inexperienced people. It is essential that units utilizing microwave energy include proper shielding in order to avoid endangering those in close promixity thereto. In order to continually monitor the effectiveness of any shielding provided, and in order to initially insure its proper installation, radiation detectors must be provided to measure any leakage radiation that may appear. The necessary measurements of the radiated power should be made in close proximity to the units being tested and yet the measuring instrument should not perturb the field. Such measurements should preferably be independent of the polarization of the incident energy field and independent of ambient temperature and infrared radiation. Furthermore, it is of importance that the monitoring device or instrument be completely reliable inasmuch as such radiation is not apparent with the use of the normal human faculties.
2. Description of the Prior Art
It has been known that thin film thermocouple elements may be used to terminate a transmission line. When connected this way, the thermocopule will be heated by an amount proportional to the power dissipated therein. This heating effect creates a voltage across the thermocouple and the value of the voltage is a direct indication of the amount of power absorbed by the thermocouple. These characteristics of thermocouples have made them a basic element in the measurement of radio frequency power.
Most measurements of microwave power are made within wave guides wherein the thermocouple can be designed and selectively positioned in order to avoid reflection of the power and wherein the environment of the thermocouples can be carefully controlled. There are no known prior developments of thermocouple probes which can be used in free space to measure microwave radiation in the near field or Fresnal region, without disturbing the field.
Thermistors or bolometers have also been used to measure radio frequency power. In some applications, for example, such elements are positioned in one leg of a bridge circuit so that the power required to maintain the bridge in balance is an indication of the amount of power aborbed by the thermistor. Such elements are generally not suitable for the type of monitor contemplated by this invention because their sensitivity is directly related to the ambient temperature and this cannot be adequately controlled.
Crystal arrangements have also been employed to indicate power density. It is known that crystals have a square law characteristic which makes them adequate for monitoring relatively low power densities. However, the restrictions of such elements to low power applications, coupled with a relatively narrow square law range, makes them unsatisfactory, for monitors of the type herein contemplated.